This invention concerns the manufacture of a tantalum capacitor anode, and more specifically the manufacture of the anode of such a capacitor having a connection wire extending therefrom.
Tantalum capacitors are notably described in copending U.S. patent application Ser. No. 933,404 filed Nov. 21, 1986 concerning a process for manufacturing a tantalum capacitor with a solid electrolyte with neither wire not output ribbon (known as a chip capacitor), a device adapted to its implementation and a capacitor thus obtained.
As is known, such a chip capacitor is a surface mount component taking the form of a short bar on the ends of which are deposited two conductive layers insulated from each other; one of these layers is connected electrically by a rod of tantalum to a porous tantalum core covered with a dielectric layer of oxide, whereas the other of these layers is in electrical contact with a counterlectrode surrounding a part of this core that is covered with a layer of solid electrolyte. This core covered with its dielectric layer, with this solid electrolyte layer, and with this counterelectrode, is encapsulated into an insulating block of resin, in the majority of cases epoxy resin.
The manufacture of such a prior art capacitor starts with the preparation of a porous body of tantalum, either cylindrical or rectangular, extended on one face by a tantalum rod. Next, a dielectric layer is formed, a layer of solid electrolyte is deposited, and then the counterelectrode is deposited. The porous body thus prepared is encapsulated in a member consisting of electrically insulating material, leaving bare a portion of the counterelectrode. Terminal conductive layers insulated from each other are deposited, respectively, in contaact with the rod and the counterelectrode. The aforementioned patent application recommends that the encapsulation be formed by molding, while masking said portion of the counterelectrode layer so as to prevent it from being completely covered.
The invention concerns in particular the preliminary phase of preparation of the porous tantalum body that is extended on one face by a tantalum wire. This porous body is also known in the capacitor art as the anode.
In the main, two techniques have been used up to now to prepare such an anode, regardless of the structures and method of manufacture of the tantalum capacitor this anode may comprise.
In accordance with a first prior art process, the anode is obtained by pressing tantalum powder to form a rectangular or cylindrical pellet; this pellet is then sintered under a vacuum at high temperature; a tantalum connection wire is electrically welded to one end. The combination of pellet and wire is then resintered under similar conditions.
In accordance with another prior art process, the anode is also obtained by pressing tantalum powder to form a rectangular or cylindrical pellet; however, the tantalum wire in this case is introduced into the pellet during pressing (embedded wire technique). The combination is then sintered in a vacuum at high temperature.
In each case, pressing and sintering occurs under adequate pressure and temperature conditions to ensure satisfactory mechanical behavior of the pellet and the wire in or on the pellet.
For high specific surface powders, it is however advisable to reduce the manufacturing temperatures and pressures of the porous tantalum pellet as much as possible in order to retain this specific surface better and to obtain a product with a high "capacity x voltage" (CV).